Using a kinetic model, we have recently completed studies on D-xylose absorption in patients with renal insufficiency, suspected malabsorption, AIDS enteropathy, and normal controls. We have shown that two rate constants characterize the absorption of D-xylose, the rate constant for absorption, ka, and the rate cons-tant for non-absorptive loss, ko. The latter is elevated in diarrheal diseases in which there is rapid transit, or bacterial overgrowth. The former rate cons-tant, ka, is low when there is a diminished functional absorptive capacity.